zlaesy (l)  Linux Man Pages
zlaesy: computes the eigendecomposition of a 2by2 symmetric matrix ( ( A, B );( B, C ) ) provided the norm of the matrix of eigenvectors is larger than some threshold value
NAME
ZLAESY  computes the eigendecomposition of a 2by2 symmetric matrix ( ( A, B );( B, C ) ) provided the norm of the matrix of eigenvectors is larger than some threshold valueSYNOPSIS
 SUBROUTINE ZLAESY(
 A, B, C, RT1, RT2, EVSCAL, CS1, SN1 )
 COMPLEX*16 A, B, C, CS1, EVSCAL, RT1, RT2, SN1
PURPOSE
ZLAESY computes the eigendecomposition of a 2by2 symmetric matrix(
ARGUMENTS
 A (input) COMPLEX*16
 The ( 1, 1 ) element of input matrix.
 B (input) COMPLEX*16
 The ( 1, 2 ) element of input matrix. The ( 2, 1 ) element is also given by B, since the 2by2 matrix is symmetric.
 C (input) COMPLEX*16
 The ( 2, 2 ) element of input matrix.
 RT1 (output) COMPLEX*16
 The eigenvalue of larger modulus.
 RT2 (output) COMPLEX*16
 The eigenvalue of smaller modulus.
 EVSCAL (output) COMPLEX*16
 The complex value by which the eigenvector matrix was scaled to make it orthonormal. If EVSCAL is zero, the eigenvectors were not computed. This means one of two things: the 2by2 matrix could not be diagonalized, or the norm of the matrix of eigenvectors before scaling was larger than the threshold value THRESH (set below).
 CS1 (output) COMPLEX*16

SN1 (output) COMPLEX*16
If EVSCAL .NE. 0, ( CS1, SN1 ) is the unit right eigenvector
for RT1.